U.S. patent number 5,340,361 [Application Number 07/975,747] was granted by the patent office on 1994-08-23 for implantable pacemaker having adaptive av interval adoptively shortened to assure ventricular pacing.
This patent grant is currently assigned to Siemens Pacesetter, Inc.. Invention is credited to Jason A. Sholder.
United States Patent |
5,340,361 |
Sholder |
August 23, 1994 |
**Please see images for:
( Certificate of Correction ) ** |
Implantable pacemaker having adaptive AV interval adoptively
shortened to assure ventricular pacing
Abstract
A dual-chamber implantable pacemaker configured to operate in
the DDD or DDDR mode automatically adjusts its AV (or PV) interval
to an amount just less than the natural conduction time of a
patient, thereby assuring that ventricular pacing occurs in a
patient's cardiac cycle at a time near when a natural ventricular
contraction (an R-wave) would occur. The pacemaker includes a pulse
generator that generates ventricular stimulation pulses (V-pulses)
at the conclusion of a pacemaker-defined AV (or PV) interval if no
natural ventricular activity (an R-wave) is sensed during such AV
(or PV) interval. The AV (or PV) intervals are automatically
adjusted by the pacemaker to be just less than the natural
conduction time sensed by the pacemaker, where the natural
conduction time is the time between atrial activity (a sensed
P-wave or a delivered A-pulse) and the subsequent natural
ventricular activity (R-wave). The system and method are
particularly adapted for use by patients suffering from a
cardiomyopathy in order to improve cardiac output.
Inventors: |
Sholder; Jason A. (Northridge,
CA) |
Assignee: |
Siemens Pacesetter, Inc.
(Sylmar, CA)
|
Family
ID: |
25523341 |
Appl.
No.: |
07/975,747 |
Filed: |
November 13, 1992 |
Current U.S.
Class: |
607/24;
607/9 |
Current CPC
Class: |
A61N
1/3627 (20130101); A61N 1/3682 (20130101) |
Current International
Class: |
A61N
1/362 (20060101); A61N 1/368 (20060101); A61N
001/36 () |
Field of
Search: |
;128/419PG |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
McAreavey, Dorothea M. D. et al., "Altered Cardiac Hemodynamic and
Electrical State in Normal Sinus Rhythm After Chronic Dual-Chamber
Pacing for Relief of Left Ventricular Outflow Obstruction in
Hypertrophic Cardiomyopathy," THE AMERICAN JOURNAL OF CARDIOLOGY,
vol. 70, pp. 651-656 (Sep. 1, 1992). .
Brecker, Stephen J. D. et al., "Effects of dual-chamber pacing with
short atriobentricular delay in dilated cardiomypathy," THE LANCET,
vol. 340, pp. 1308-1312 (Nov. 28, 1992). .
Barold, S. Serge MD, BS, FRACP, "Cardiac Pacing in Special and
Complex Situations," CARDIAC PACING, vol. 10, No. 4, pp. 573-591
(Nov. 1992). .
Hochleitner, Margarete M. D. et al., "Long-Term Efficacy of
Physiologic Dual-Chamber Pacing in the Treatment of End-Stage
Idiopathic Dilated Cardiomyopathy," THE AMERICAN JOURNAL OF
CARDIOLOGY, vol. 70, pp. 1320-1325, (Nov. 15, 1992). .
Hochleitner, Margarete M. D. et al., "Usefulness of Physiologic
Dual-Chamber Pacing in Drug-Resistant Idiopathic Dilated
Cardiomyopathy,"THE AMERICAN JOURNAL OF cARDIOLOGY, vol. 66, pp.
198-202 (Jul. 15, 1990,)..
|
Primary Examiner: Kamm; William E.
Assistant Examiner: Getzow; Scott M.
Attorney, Agent or Firm: Romano; Malcolm J. Weinberg; Lisa
P.
Claims
What is claimed is:
1. A dual-chamber pacemaker providing atrioventricular pacing in
order to increase cardiac output in a patient suffering from a
cardiomyopathy, comprising:
an atrial channel and a ventricular channel;
an atrial sense amplifier that senses a P-wave in the atrial
channel, said P-wave representing natural atrial activity;
a ventricular sense amplifier that senses an R-wave in the
ventricular channel, said R-wave representing natural ventricular
activity;
pulse generator means for generating an atrial stimulation pulse
(A-pulse) in the atrial channel in the absence of a sensed P-wave
by said atrial sense amplifier within an atrial escape interval,
and a ventricular stimulation pulse (V-Pulse) in the ventricular
channel in the absence of a sensed R-wave by said ventricular sense
amplifier within an AV time interval;
a control system that defines said AV time interval and said atrial
escape interval, said AV time interval beginning upon the sensing
of a P-wave or the generation of an A-pulse, whichever event occurs
first in the atrial channel, and said atrial escape interval
beginning upon the sensing of an R-wave or the generation of a
V-pulse, whichever event occurs first in the ventricular channel;
and
timing means as part of said control system for measuring a
conduction time interval as the time period between atrial activity
in the atrial channel and the sensing of an R-wave in the
ventricular channel, and for automatically setting said AV time
interval to a value that for ventricular pacing is always less than
the measured conduction time interval by a prescribed amount, said
atrial activity comprising the generation of an A-pulse or the
sensing of a P-wave, whichever event occurs first, in the atrial
channel;
whereby, in the absence of a decreasing conduction time interval,
said pulse generator means generates said V-pulse in the
ventricular channel prior to the occurrence of an R-wave, thereby
providing ventricular pacing for increasing cardiac output; and
further
whereby, in the presence of a decreasing conduction time interval,
said AV time interval is automatically adjusted to a value less
than the shortest conduction time interval.
2. The dual-chamber pacemaker, as set forth in claim 1, wherein
said timing means of said control system further automatically
increases said AV time interval by a prescribed amount in the event
a prescribed number of consecutive cardiac cycles occur without an
R-wave being sensed by said ventricular sense amplifier; a cardiac
cycle comprising the time period between consecutive atrial
activity, whereby said AV time interval does not remain adjusted to
a value less than the shortest conduction time interval in the
absence of sensed R-waves for a period of time longer than said
prescribed number of cardiac cycles.
3. The dual-chamber pacemaker, as set forth in claim 2, further
comprising a memory circuit coupled to said control system, said
memory circuit having prescribed minimum and maximum AV interval
values stored therein, said minimum and maximum AV interval values
being usable by said control system to limit the adjustment of said
AV time interval between said minimum and maximum AV interval
values.
4. The dual-chamber pacemaker, as set forth in claim 3, wherein
said control system comprises a state logic circuit that changes
states in response to the occurrence of prescribed events, said
prescribed events including the timing-out of specified time
intervals, and the sensing of activity in the atrial or ventricular
channels.
5. The dual-chamber pacemaker, as set forth in claim 3, wherein
said control system comprises a programmable timer circuit and a
clock circuit, said programmable timer circuit having a time
interval value loaded therein, said clock circuit generating a
clock signal that counts said programmable timer circuit for the
number of counts represented by said time interval value, whereupon
said programmable timer circuit issues a time-out signal; said time
interval value being adaptively changed by logic circuits within
said control system to represent the AV time interval, whereby said
programmable timer circuit defines said AV time interval.
6. A dual-chamber implantable pacemaker programmed to operate in a
DDD or DDDR mode of operation so as to generate a ventricular
stimulation pulse (V-pulse) during a given cardiac cycle prior to
natural ventricular activity, yet at a time delayed sufficiently
from any preceding atrial activity to assure efficient cardiac
output, said pacemaker comprising:
sensing means for sensing P-waves and R-waves;
stimulation means for generating V-pulses and A-pulses;
measurement means for measuring the natural conduction time,
t.sub.PR, or t.sub.AR, of a heart to which the pacemaker is
coupled, where t.sub.PR represents the natural conduction time
following a natural atrial event (P-wave), and t.sub.AR represents
the natural conduction time of the heart following an atrial
stimulation pulse (A-pulse); and
timing means for automatically setting a PV interval for use by
said pacemaker following a P-wave, and an AV interval for use by
said pacemaker following an A-pulse, to a prescribed amount less
than said natural conduction time, t.sub.PR or t.sub.AR,
respectively, said PV and AV intervals being used by said timing
means to define the time period between a P-wave and a V-pulse, and
between an A-pulse and a V-pulse, respectively, during the
operation of said pacemaker.
7. The dual-chamber implantable pacemaker, as set forth in claim 6,
wherein said timing means automatically adjusts said PV and AV
intervals to make them a prescribed amount less than the PV or AV
interval used by said pacemaker in a most recent cardiac cycle in
the event an R-wave is sensed during said most recent pacing
cycle.
8. The dual-chamber implantable pacemaker, as set forth in claim 7,
wherein said timing means further automatically adjusts said PV and
AV intervals to make them a prescribed amount greater than the PV
or AV interval used by said pacemaker in a most recent cardiac
cycle, up to a predetermined maximum PV or AV interval, in the
event no R-wave is sensed for a specified number of consecutive
prior cardiac cycles.
9. The dual-chamber implantable pacemaker, as set forth in claim 6,
wherein said measurement means includes means for measuring one of
said t.sub.PR or t.sub.AR natural conduction times, depending upon
which of said A-pulse or P-wave occurs first, and computing the
other of said t.sub.PR or t.sub.AR natural conduction times as a
function of said measured one of said t.sub.PR or t.sub.AR natural
conduction times.
10. The dual-chamber implantable pacemaker, as set forth in claim
9, wherein said means for computing the other of said t.sub.PR or
t.sub.AR natural conduction times once one has been measured
comprises means for defining t.sub.AR to be a prescribed amount
greater than t.sub.PR, whereby once one of said t.sub.PR or
t.sub.AR natural conduction times has been measured, the other of
said t.sub.PR or t.sub.AR natural conduction times is computed to
be less than or greater than the measured value of t.sub.PR or
t.sub.AR by said prescribed amount.
11. The dual-chamber implantable pacemaker, as set forth in claim
6, wherein said measurement means includes means for measuring both
t.sub.PR and t.sub.AR, where t.sub.PR is measured as the time
interval between the occurrence of a P-wave and a subsequent
R-wave, and t.sub.AR is measured as the time interval between the
generation of an A-pulse and a subsequent R-wave.
12. The dual-chamber implantable pacemaker, as set forth in claim
6, wherein said timing means sets the PV interval to be a first
prescribed amount less than t.sub.PR, and sets the AV interval to
be a second prescribed amount less than t.sub.AR.
13. The dual-chamber implantable pacemaker, as set forth in claim
12, wherein said first prescribed amount is the same as said second
prescribed amount, whereby the difference between said PV interval
and said AV interval is the same as the difference between t.sub.AR
and t.sub.PR.
14. The dual-chamber implantable pacemaker, as set forth in claim
12, wherein said first prescribed amount and said second prescribed
amount are computed as a function of the most recent measured
values of t.sub.PR and t.sub.AR, respectively.
15. A dual chamber pacemaker comprising:
means for sensing and pacing in both an atrial and ventricular
channel;
timing means for setting an AV interval, said AV interval
comprising the maximum time period allowed by the pacemaker between
atrial channel activity, which atrial channel activity includes
sensing or pacing in the atrial channel, and pacing in the
ventricular channel;
measurement means for measuring a first time interval as the time
interval between atrial channel activity and sensing in the
ventricular channel;
adjustment means for automatically setting the AV interval of said
pacemaker to a value that for pacing in the ventricular channel is
always less than the most recently measured first time interval;
and
control means for pacing in the ventricular channel at the
conclusion of said AV interval in the event that no sensing occurs
in the ventricular channel during said AV interval, whereby
ventricular pacing is provided.
16. The dual-chamber pacemaker, as set forth in claim 15, wherein
said measurement means measures said first time interval each time
that sensing occurs in the ventricular channel.
17. The dual-chamber pacemaker, as set forth in claim 15, wherein
said adjustment means also increases said AV interval by a
specified amount in the event that no sensing occurs in the
ventricular channel for a prescribed number of cardiac cycles,
where a cardiac cycle comprises the consecutive occurrence of
atrial channel activity followed by pacing in the ventricular
channel.
18. A method of operating a dual-chamber implantable pacemaker to
provide ventricular stimulation pulses at a time within a cardiac
cycle that is just prior to when a natural ventricular contraction
would occur, said implantable pacemaker including means for sensing
natural atrial contractions (P-waves), means for sensing natural
ventricular contractions (R-waves), means for generating atrial
stimulation pulses (A-pulses), means for generating ventricular
stimulation pulses (V-pulses), and timing means for defining an AV
interval that commences with the generation of an A-pulse, and a PV
interval that commences with the sensing of a P-wave, said method
comprising:
(a) electronically measuring a first time interval within a given
cardiac cycle that comprises the time between an atrial event and
an R-wave, said atrial event comprising either an A-pulse or a
P-wave, whichever event occurs first in the given cardiac
cycle;
(b) automatically setting said AV interval to be equal to said
first time interval less a first prescribed amount;
(c) automatically setting said PV interval to be equal to said
first time interval less a second prescribed amount; and
(d) issuing a V-pulse at the conclusion of said AV interval, in the
event an A-pulse has first been generated, or at the conclusion of
said PV interval, in the event a P-wave has first been sensed;
whereby said V-pulse is always generated at the conclusion of said
AV or PV interval following said atrial event.
19. The method, as set forth in claim 18, wherein steps (a) to (c)
are performed periodically at a prescribed interval, and step (d)
is performed every cardiac cycle using the most recent AV or PV
interval set by steps (b) and (c).
20. The method, as set forth in claim 19, wherein the prescribed
interval at which steps (a)-(c) are performed comprises an interval
defined by the occurrence of a programmed number of consecutive
cardiac cycles.
21. The method, as set forth in claim 18, wherein steps (a) to (c)
are performed during each cardiac cycle in which an R-wave
occurs.
22. The method, as set forth in claim 21, further including
increasing said AV and PV intervals by a third prescribed amount up
to a maximum AV and PV interval in the event a predetermined number
of consecutive cardiac cycles occurs without the occurrence of an
R-wave.
23. In a dual-chamber implantable pacemaker programmed to operate
in a DDD or DDDR mode of operation, a method of operating said
pacemaker when coupled to a heart of a patient so as to generate a
ventricular stimulation pulse (V-pulse) during a given cardiac
cycle prior to natural ventricular activity, yet at a time delayed
sufficiently from any preceding atrial activity to assure efficient
cardiac output, said pacemaker including means for sensing P-waves
and R-waves, and means for generating V-pulses and A-pulses, said
method comprising the steps of:
(a) determining the natural conduction time of the heart, t.sub.PR
or t.sub.AR, where t.sub.PR represents the natural conduction time
following a natural atrial event (P-wave), and t.sub.AR represents
the natural conduction time of the heart following an atrial
stimulation pulse (A-pulse);
(b) automatically setting a PV interval, for use by said pacemaker
following a P-wave, and an AV interval, for use by said pacemaker
following an A-pulse, to a prescribed amount less than said natural
conduction time, t.sub.PR or t.sub.AR, respectively;
(c) performing at least one pacing cycle in said DDD or DDDR mode
of operation using the PV and AV intervals to define the time
period between an atrial event and the generation of a V-pulse;
(d) automatically adjusting said PV and AV intervals to make them a
prescribed amount less than the PV or AV interval used most
recently by said pacemaker in the event an R-wave is sensed during
said at least one pacing cycle; and
(e) automatically adjusting said PV and AV intervals to make them a
prescribed amount greater than the PV or AV interval used most
recently by said pacemaker, up to a predetermined maximum PV or AV
interval, in the event no R-wave is sensed for a specified number
of cardiac cycles.
24. The method, as set forth in claim 23, wherein step (a)
comprises measuring one of said t.sub.PR or t.sub.AR natural
conduction times, depending upon which of said A-pulse or P-wave
occurs first, and computing the other of said t.sub.PR or t.sub.AR
natural conduction times as a function of said measured one of said
t.sub.PR or t.sub.AR natural conduction times.
25. The method, as set forth in claim 24, wherein computing the
other of said t.sub.PR or t.sub.AR natural conduction times once
one has been measured comprises defining t.sub.AR to be a
prescribed amount greater than t.sub.PR, whereby once one of said
t.sub.PR or t.sub.AR natural conduction times has been measured,
the other of said t.sub.PR or t.sub.AR natural conduction times is
computed to be less than or greater than the measured value of
t.sub.PR or t.sub.AR by said prescribed amount.
26. The method, as set forth in claim 23, wherein step (a)
comprises measuring both t.sub.PR and t.sub.AR, where t.sub.PR is
measured as the time interval between the occurrence of a P-wave
and a subsequent R-wave, and t.sub.AR is measured as the time
interval between the generation of an A-pulse and a subsequent
R-wave.
27. The method, as set forth in claim 23, wherein step (b)
comprises setting the PV interval to be a first prescribed amount
less than t.sub.PR, and setting the AV interval to be a second
prescribed amount less than t.sub.AR.
28. The method, as set forth in claim 27, wherein said first
prescribed amount is the same as said second prescribed amount,
whereby the difference between said PV interval and said AV
interval is the same as the difference between t.sub.AR and
t.sub.PR.
29. The method, as set forth in claim 27, wherein said first
prescribed amount and said second prescribed amount are computed as
a function of the most recent measured values of t.sub.PR and
t.sub.AR, respectively.
30. A method of providing ventricular pacing in order to increase
cardiac output in a cardiomyopathy patient, such patient having a
dual-chamber pacemaker, said pacemaker including means for sensing
and pacing in both an atrial and ventricular channel, and timing
means for setting an AV interval, said method comprising the steps
of:
(a) measuring a first time interval as the time interval between
atrial channel activity and sensing in the ventricular channel for
determining a natural conduction time, said atrial channel activity
comprising sensing or pacing in the atrial channel;
(b) automatically setting the AV interval of said pacemaker to a
value that is a prescribed amount less than the natural conduction
time, where said AV interval defines the time interval between
atrial channel activity and pacing in the ventricular channel;
and
(c) pacing in the ventricular channel at the conclusion of said AV
interval.
31. The method, as set forth in claim 30, further comprising the
step of:
(d) adjusting said AV interval to a value that is said prescribed
amount less than the time interval between the most recent atrial
channel activity and sensing in the ventricular channel in the
event that sensing occurs in the ventricular channel during said AV
interval.
32. The method, as set forth in claim 31, further comprising the
step of:
(e) increasing said AV interval by a specified amount in the event
that no sensing occurs in the ventricular channel for a prescribed
number of cardiac cycles, where a cardiac cycle comprises the
consecutive occurrence of atrial channel activity followed by
pacing in the ventricular channel.
33. A dual-chamber pacemaker for controlling ventricular pacing in
order to increase cardiac output in a patient suffering from a
cardiomyopathy by preemptively stimulating the ventricular channel
comprising:
an atrial channel and a ventricular channel;
an atrial sense amplifier that senses a P-wave in the atrial
channel, said P-wave representing natural atrial activity;
a ventricular sense amplifier that senses an R-wave in the
ventricular channel, said R-wave representing natural ventricular
activity;
pulse generator means for generating a ventricular stimulation
pulse (V-Pulse) in the ventricular channel and an atrial
stimulation pulse (A-pulse) in the atrial channel, the sensing of a
P-wave or the generating of an A-pulse, whichever occurs first,
comprising atrial activity; and
timing means for defining an AV time interval as the time interval
between atrial activity and the generation of a V-pulse, said
timing means further for measuring a natural conduction time
interval as the time period between atrial activity and the sensing
of an R-wave, and for automatically setting said AV time interval
to a value that is less than said natural conduction time interval,
whereby said pulse generator generates said V-pulse prior to the
occurrence of natural ventricular activity for preemptively
stimulating the ventricular channel to thereby increase cardiac
output.
34. The dual-chamber pacemaker of claim 33, further comprising
means for setting the AV interval to a value that, for ventricular
pacing, is always less than the natural conduction time interval by
a prescribed amount.
Description
BACKGROUND OF THE INVENTION
Field of the Invention
The present invention relates to implantable medical devices and
methods, and more particularly, to an implantable pacemaker that
automatically adapts its atrial-ventricular (AV) delay to maximize
the cardiac output for patients having a cardiomyopathy.
The heart is a pump that pumps life-sustaining blood throughout the
body of the patient. The human heart comprises a left side and a
right side, with each side having a first chamber, known as the
atrium, and a second chamber, known as the ventricle. The atrium
receives blood returning from other body locations. At an
appropriate time, determined by the sinoatrial (SA) node, an
electrical stimulus is provided that causes the muscle tissue
surrounding the atrium to depolarize. Depolarization of the atrial
muscle tissue is manifest by the occurrence of an electrical signal
known as the P-wave. Immediately following the P-wave, the atrial
muscle tissue physically contracts, forcing the blood held in the
atrium through a one-way valve into the ventricle. The SA node
stimulus that caused the atrium to depolarize also travels to the
ventricle through the atrioventricular (AV) node and the
atrioventricular (AV) bundle. The AV node is a mass of modified
heart muscle situated in the lower middle part of the right atrium.
It receives the impulse to contract from the sinoatrial node, via
the atria, and transmits it through the atrioventricular bundle to
the ventricles. The AV bundle is a bundle of modified heart muscle
fibers (Purkinje fibers) that pass from the AV node forward to the
septum between the ventricles, where it divides into right and left
bundles, one for each ventricle. The fibers thus transmit the SA
node stimulus from the atria, via the AV node, to the ventricles.
However, as the SA node stimulus travels through the AV bundle, it
is delayed by an amount commensurate with the same time it takes
the blood to physically flow from the atrium to the ventricle.
After the delay through the AV bundle, which delay may be referred
to as the natural conduction time of the heart, the SA node
stimulus arrives at the ventricular muscle tissue, causing it to
depolarize. Depolarization of the ventricular muscle tissue is
manifest by the occurrence of an electrical signal known as the
R-wave (sometimes referred to as the QRS complex). Immediately
following the R-wave, the ventricular muscle tissue physically
contracts, forcing the blood held therein through one or more
arteries to various body locations. In this manner, then, the heart
"beats" or pumps blood by having the atria contract at a rate
determined by the SA node, and after the natural conduction time,
by having the ventricles contract. After a longer delay, when the
atrium has refilled with blood returning from throughout the body,
the process repeats.
The heart of a typical healthy patient may beat 60-70 times per
minute when the patient is at rest. When the patient is undergoing
significant physiological stress, as occurs, e.g., during physical
exercise, the rate at which the heart beats, the "heart rate,"
increases significantly, e.g, up to 150-170 times per minute. The
above-described process wherein the atria and ventricles
sequentially depolarize and contract in order to pump blood, and
get ready to depolarize again, is referred to herein as the
"cardiac cycle." A given cardiac cycle thus includes one R-wave (or
equivalent ventricular activity evidencing depolarization of the
ventricles) and one P-wave (or equivalent atrial activity
evidencing depolarization of the atria). The length of the cardiac
cycle (which represents the period of the heart rate) may be
measured as the time interval between successive P-waves or
R-waves, although R-waves are usually used because they are easier
to detect.
A pacemaker is an implantable medical device that monitors the
activity of the heart for the occurrence of P-waves and/or R-waves,
and steps in with electronically generated stimuli, when needed, to
force the depolarization of the atria and/or ventricles. A
pacemaker-generated stimulus that is delivered to the atrium is
referred to herein as an "A-pulse." A pacemaker-generated stimulus
that is delivered to the ventricle is referred to herein as a
"V-pulse." Most pacemakers are configured to provide an A-pulse
and/or V-pulse only if a prescribed period of time has elapsed
without the occurrence of a P-wave and/or an R-wave, i.e., without
the occurrence of natural heart beats.
The prescribed period of time used by the pacemaker between
contraction of the ventricle and contraction of the atrium is
generally referred to as the V-A Interval, or the atrial escape
interval. For most dual-chamber pacemaker modes of operation, only
if a P-wave does not occur during the atrial escape interval will
the pacemaker step in at the conclusion of such interval and
generate an A-pulse.
The prescribed period of time used by the pacemaker between
contraction of the atrium and contraction of the ventricle is
referred to as the A-V Interval, or sometimes it is called the "AV
Delay." The pacemaker, for most dual-chamber modes of operation,
generates a V-pulse only if the AV Delay elapses without the
occurrence of an R-wave.
In the above-described manner, the heart is thus afforded as much
time as possible to beat on its own before the
electronically-generated stimuli of the pacemaker are delivered to
the heart, causing it to beat at the rate set by the pacemaker.
Heretofore, most cardiac patients using a pacemaker have suffered
from at least one of various cardiac conditions or diseases that
affect either the ability of the SA node to maintain and sustain a
satisfactory heart beat rate (hereafter "rate problems"), or the
ability of the AV node or the AV bundle to conduct a suitable
stimulus to the ventricle (hereafter "conduction problems").
Advantageously, both rate problems and conduction problems lend
themselves well to a pacemaker solution because the underlying
cardiac muscle tissue is in place and is capable of responding to
the electronically-generated stimuli produced by the pacemaker.
Unfortunately, there remain a significant number of patients that
suffer from one or more conditions that cannot be characterized as
either rate problems or conduction problems. One such problem is
known as hypertrophic cardiomyopathy. Another is known as dilated
cardiomyopathy. While there are medical or clinical differences
between these two forms of cardiomyopathy, for purposes of the
present invention they may be considered the same problem, and will
be referred to hereafter as simply "cardiomyopathy."
In general, a patient suffering from cardiomyopathy experiences a
significant reduction in cardiac output because the heart muscle is
unable to perform its function of contracting in response to the SA
node stimulus. By "cardiac output," it is meant the ability of the
heart to efficiently pump blood. Thus, a patient suffering from
cardiomyopathy will generally not have as much blood pumped per
heart beat (stroke volume) as may be needed. Cardiomyopathy
patients are referred to as being moderately to severely
symptomatic of low cardiac output syndrome. The only treatment for
low cardiac output syndrome, up to now, has been heart
transplantation. Disadvantageously, heart transplantation is not a
viable solution for most patients. Not only are hearts suitable for
transplant difficult and expensive to secure, but even when
secured, a very dangerous and complicated surgery must follow in
order to successfully perform the transplantation operation. What
is thus needed is an alternative to heart transplantation for
patients suffering from low cardiac output syndrome.
It has recently been proposed to implant a dual-chamber pacemaker
in patients suffering from low cardiac output syndrome and to
configure such pacemaker to provide PV or AV pacing. During PV or
AV pacing, the pacemaker delivers a V-pulse to the ventricles a
programmed delay after the occurrence of an atrial event, which
atrial event could be either the occurrence of a P-wave or the
delivery of an A-pulse. Advantageously, by forcing a ventricular
contraction prior to the occurrence of an R-wave, i.e., prior to
natural depolarization of the ventricles, the cardiac output of
patients suffering from cardiomyopathies may be significantly
improved. Such improvement appears to result because the
ventricular stimulus--a V-pulse delivered by the pacemaker--is
applied to the ventricular tissue at a different cardiac location
(at the location of the ventricular lead tip electrode, which
location is usually in the apex of the right ventricle) than is the
natural stimulus when received through the SA node.
PV or AV pacing is only effective, however, when the V-pulse is
delivered to the ventricular tissue before the occurrence of an
R-wave, i.e., before the ventricular tissue depolarizes. As soon as
the ventricular tissue depolarizes, it becomes refractory, and will
not respond to a V-pulse, until such time as it repolarizes. It is
thus necessary, if AV or PV pacing is to be used, to set the AV (or
PV) interval of the pacemaker to a value that is less than the
patient's normal conduction time. Unfortunately, heretofore, this
requirement has forced the AV (or PV) interval to be set to very
short values, i.e., between 80 and 120 msec, because during
exercise (or other periods of physical activity or physiological
stress) the patient's native conduction time may shorten
significantly. Thus, in order to guarantee that the pacemaker will
always pace the ventricles, i.e., in order to assure that the
V-pulse is delivered to the ventricular tissue at a time when it is
not refractory, the AV (or PV) interval must be set to an interval
that is shorter than any native conduction interval that might
exist in any given patient at any given time.
Disadvantageously, setting a very short programmed AV (or PV)
interval may adversely affect cardiac output because it may force
ventricular contraction well before the ventricles have had
sufficient time to be filled with blood from the atrium. Thus, what
is needed for patients suffering from a cardiomyopathy is a
pacemaker that paces the ventricles at a time in the cardiac cycle
that is always less than the natural conduction time, i.e., at a
time that is prior to the occurrence of an R-wave, but that is not
so much less than the natural conduction time so as to adversely
affect cardiac output. That is, what is needed is a pacemaker that
automatically sets its internally-generated AV and/or PV intervals
to be just short of the patient's native conduction time, thereby
assuring that the AV (or PV) interval is sufficiently long to allow
the blood to physically move from the atrium to the ventricles; yet
remains sufficiently short to always be less than the patient's
native conduction time, thereby assuring that the V-pulse is not
delivered when the ventricular tissue is refractory.
The present invention advantageously addresses the above and other
needs. See Applicant's copending application, filed concurrently
herewith, entitled DUAL-CHAMBER IMPLANTABLE PACEMAKER HAVING AN
ADAPTIVE AV INTERVAL THAT PREVENTS VENTRICULAR FUSION BEATS, Ser.
No. 07/976,153, Attorney Docket No. GR 92P 7968, which application
is incorporated herein by reference.
SUMMARY OF THE INVENTION
The present invention provides a dual-chamber implantable pacemaker
and a method of operating such a dual-chamber implantable
pacemaker, wherein the natural conduction time of a patient is
measured, and the AV (or PV) interval of a dual-chamber pacemaker
implanted in the patient is automatically set to a value just less
than the measured natural conduction time. A ventricular
stimulation pulse (V-pulse) is generated at the conclusion of the
pacemaker-defined AV (or PV) interval if no natural ventricular
activity (an R-wave) is sensed during such AV (or PV) interval.
Because the AV (or PV) interval is automatically set to a value
just less than the natural conduction time, a V-pulse will almost
always be applied to the ventricular muscle tissue at a time when
such muscle tissue is capable of responding thereto, i.e., at a
time when the tissue is not refractory. In the event that an R-wave
does occur, signaling that the natural conduction time of the
patient is decreasing (as might occur, for example, if the patient
is exercising), the occurrence of the R-wave provides a new measure
of the natural conduction time, which thereafter affords a basis
for further adjusting the AV interval.
The pacemaker-defined AV interval begins upon the delivery of an
atrial stimulation pulse (A-pulse) by the pacemaker. Similarly, the
pacemaker-defined PV interval begins upon sensing natural atrial
activity (a P-wave) by the pacemaker. The natural conduction time
measured by the pacemaker comprises the time between atrial
activity (whether a sensed P-wave or a delivered A-pulse, whichever
occurs) and subsequent natural ventricular activity (an R-wave).
The method of operating a pacemaker in accordance with the present
invention thus includes: (1) measuring the natural conduction time,
t.sub.AR, of the patient in a given cardiac cycle; and (2) setting
the AV (or PV) interval of the pacemaker, for use in subsequent
cardiac cycles, to a value that is a prescribed amount, e.g, 20-30
msec., less than t.sub.AR.
In accordance with one aspect of the invention, the pacemaker
includes a timing counter, or equivalent, that is initiated upon
the occurrence of each atrial event, whether a P-wave or an
A-pulse. The atrial event also starts the AV (or PV) interval of
the pacemaker. If an R-wave occurs in the cardiac cycle before the
termination of the AV (or PV) interval, then the timing counter
stops, with the count held therein providing a measure of the
natural conduction time, t.sub.AR. The AV (or PV) interval set by
the pacemaker is then immediately and automatically adjusted to a
new value that is the prescribed amount less than t.sub.AR. The new
adjusted value of AV (or PV) is then used for the next cardiac
cycle. In this manner, the AV (or PV) interval is adaptively
adjusted, as required, to always be less than the natural
conduction time of the patient.
In accordance with another aspect of the invention, if a prescribed
number of consecutive cardiac cycles ensue without the occurrence
of an R-wave, then the value of the AV (or PV) interval is
gradually increased, in order to incrementally return it to its
original value.
A dual-chamber pacemaker made in accordance with the present
invention includes an atrial channel and a ventricular channel. An
atrial sense amplifier senses the occurrence of natural atrial
activity (a P-wave) in the atrial channel. A ventricular sense
amplifier similarly senses the occurrence of natural ventricular
activity (an R-wave) in the ventricular channel. An atrial pulse
generator generates an atrial stimulation pulse (A-pulse) in the
atrial channel in the absence of a sensed P-wave by the atrial
sense amplifier within an AV time interval. Similarly, a
ventricular pulse generator generates a ventricular stimulation
pulse (V-Pulse) in the ventricular channel in the absence of a
sensed R-wave by the ventricular sense amplifier within an atrial
escape interval. A control circuit coupled to both the atrial and
ventricular channels defines the AV time interval and the atrial
escape interval. The AV time interval begins upon the sensing of
atrial activity in the atrial channel, where atrial activity may be
either a P-wave or the generation of an A-pulse, whichever event
occurs. The atrial escape interval begins upon the sensing of
ventricular activity in the ventricular channel, where ventricular
activity may be either an R-wave or the generation of a V-pulse,
whichever event occurs first. The control circuit of the pacemaker
includes timing means for measuring a natural conduction time
interval as the time period between atrial activity in the atrial
channel and the sensing of an R-wave in the ventricular channel. In
accordance with the present invention, the control circuit
automatically decreases the AV time interval to a value that is
less than the natural conduction time interval by a prescribed
amount, which decreased AV time interval value is not to be less
than a minimum AV time interval value.
Hence, in the absence of a decreasing natural conduction time
interval, the pacemaker of the present invention generates a
V-pulse in the ventricular channel prior to the occurrence of an
R-wave, thereby providing needed therapy for patients who most
always need a V-pulse, e.g., patients suffering from a
cardiomyopathy. Further, in the presence of a decreasing natural
conduction time interval, the pacemaker of the invention
automatically decreases the AV time interval to a value that is
less than the shortest conduction time interval.
Moreover, in accordance with one aspect of the invention, the
control circuit of the dual-chamber pacemaker automatically
increases the AV time interval by a prescribed amount in the event
a prescribed number of consecutive cardiac cycles occur without an
R-wave having been sensed by the ventricular sense amplifier. Thus,
the AV time interval will never remain adjusted to a value less
than the shortest conduction time interval in the absence of sensed
R-waves for a period of time longer than the prescribed number of
cardiac cycles. In this manner, then, the pacemaker adaptively
adjusts its AV time interval, as required, between maximum and
minimum values, always attempting to provide a V-pulse just prior
to when an R-wave would otherwise occur.
It is thus a feature of the present invention to provide an
implantable pacemaker and method of operating such a pacemaker that
stimulates cardiac tissue at a time in the cardiac cycle that is
just prior to when natural depolarization of the cardiac tissue
would otherwise cause a cardiac contraction.
It is another feature of the invention to provide a dual-chamber
pacemaker, and method of operating such a dual-chamber pacemaker,
that automatically adjusts its pacemaker-defined AV interval to a
value that is just less than the natural conduction time of a
patient, thereby assuring that a V-pulse is generated and delivered
to the ventricular muscle tissue at a time in the cardiac cycle
when such ventricular muscle is not refractory (i.e., prior to the
natural depolarization of the ventricular tissue), while still
maintaining the approximate cardiac timing set by the natural
conduction time, whereby the cardiac output of the patient is
maximized.
It is a further feature of the invention to provide such a
pacemaker, and method of operating such a pacemaker, that decreases
the pacemaker-defined AV interval in response to sensing an R-wave
(which sensed R-wave evidences a shortened natural conduction
time), and that automatically increases the pacemaker-defined AV
interval in prescribed increments in response to not sensing an
R-wave for a prescribed number of consecutive cardiac cycles (which
failure to sense any R-waves may evidence a lengthening of the
natural conduction time).
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other aspects, features and advantages of the present
invention will become more apparent from the Detailed Description
of the Invention, presented in conjunction with the following
drawings, wherein:
FIG. 1 is block diagram of a dual-chamber programmable
pacemaker;
FIG. 2 is a block diagram of one embodiment of the control logic of
the pacemaker of FIG. 1;
FIG. 3 is a flowchart illustrating the method of the present
invention;
FIG. 4 is a more detailed flowchart illustrating the method of the
present invention;
FIG. 5 is a flowchart that illustrates one technique for measuring
the natural conduction time of a patient; and
FIG. 6 is a flowchart that illustrates another embodiment for
determining the natural conduction time of a patient, wherein one
of the PR or AR conduction time intervals is measured, and the
other is set as a prescribed difference from the measured
value.
Corresponding reference characters indicate corresponding
components throughout the several views of the drawings.
DETAILED DESCRIPTION OF THE INVENTION
The following description is of the best mode presently
contemplated for carrying out the invention. This description is
not to be taken in a limiting sense, but is made merely for the
purpose of describing the general principles of the invention. The
scope of the invention should be determined with reference to the
claims.
As indicated above, the present invention is directed to an
implantable dual-chamber pacemaker, and a method of operating an
implantable dual-chamber pacemaker, that automatically adapts or
adjusts the AV interval (or PV interval) of the pacemaker in an
attempt to maximize the cardiac output of a patient suffering from
a cardiomyopathy. In general, the muscle tissue (usually the
ventricular muscle tissue) of the heart of a patient suffering from
a cardiomyopathy is unable to provide a strong beat (muscle
contraction), and is thus not able to efficiently pump much blood
with each beat. If a ventricular stimulation pulse (V-pulse) is
provided to the heart at the right time in the cardiac cycle, then
a stronger beat (muscle contraction) is provided, and the cardiac
output (amount of blood pumped by the heart) of the patient
increases. The "right time" to provide such a V-pulse in the
cardiac cycle is just prior to when the ventricles would beat
(depolarize, and hence contract) on their own due to the normal
conduction time of the patient, i.e., just prior to the occurrence
of an R-wave. To this end, the present invention determines the
natural conduction time between a P-wave (evidencing depolarization
of the atria) and a subsequent R-wave, or PR interval, and sets the
PV interval of the pacemaker to be a prescribed amount less than
such PR interval. Alternatively, should the atria of the patient
also require stimulation, the invention determines the paced
conduction time between an atrial stimulation pulse, (A-pulse) and
a subsequent R-wave, or AR interval, and sets the AV interval of
the pacemaker to be a prescribed amount less than such AR interval.
In this manner, the pacemaker always delivers a V-pulse at the
conclusion of the PV or AV intervals, which is less than the
natural conduction time (PR or AR interval), and hence before the
ventricles attempt to contract on their own.
Advantageously, the present invention may be implemented using a
wide variety of dual-chamber pacemaker configurations and pacemaker
hardware. Any pacemaker configuration that allows the pacemaker AV
or PV intervals to be automatically set to a value that is a
prescribed amount less than the AR or PR conduction-time intervals
may be used to implement the invention. The description that
follows is only exemplary of one such configuration.
Referring then to FIG. 1, a block diagram of a dual-chamber
pacemaker 10 is illustrated. The pacemaker 10 is coupled to a heart
12 by way of leads 14 and 16. The lead 14 has an electrode 15 that
is in contact with one of the atria of the heart, and the lead 16
has an electrode 17 that is in contact with one of the ventricles
of the heart. The leads 14 and 16 carry stimulating pulses to the
electrodes 15 and 17 from an atrial pulse generator (A-PG) 18 and a
ventricular pulse generator (V-PG) 20, respectively. Further,
electrical signals from the atria are carried from the electrode
15, through the lead 14, to the input terminal of an atrial channel
sense amplifier (P-AMP) 22; and electrical signals from the
ventricles are carried from the electrode 17, through the lead 16,
to the input terminal of a ventricular sense channel amplifier
(R-AMP) 24.
A control circuit or control system 26 controls the dual-chamber
pacer 10. The control system 26 receives the output signals from
the atrial amplifier 22 over signal line 28. Similarly, the control
system 26 receives the output signals from the ventricular
amplifier 24 over signal line 30. The output signals on signal
lines 28 and 30 are generated each time that a P-wave or an R-wave
is sensed within the heart 12. The control circuit or system 26
also generates trigger signals that are sent to the atrial pulse
generator 18 and the ventricular pulse generator 20 over signal
lines 32 and 34, respectively. These trigger signals are generated
each time that a stimulation pulse is to be generated by the
respective pulse generator 18 or 20. A stimulation pulse generated
by the A-PG 18 is referred to as the "A-pulse," and the stimulation
pulse generated by the V-PG 20 is referred to as the "V-pulse."
During the time that either an A-pulse or V-pulse is being
delivered to the heart, the corresponding amplifier, P-AMP 22
and/or R-AMP 24, is typically disabled by way of a blanking signal
presented to these amplifiers from the control system over signal
lines 36 and 38, respectively. This blanking action prevents the
amplifiers 22 and 24 from becoming saturated from the relatively
large A-pulse or V-pulse, respectively, that is present at the
input terminals of such amplifiers during this time. Such blanking
action also prevents the sensing of residual electrical signals
that may be present in the muscle tissue as a result of the pacer
stimulation, which sensing could falsely be interpreted as P-waves
or R-waves.
Still referring to FIG. 1, the pacer 10 also includes a memory
circuit 40 that is coupled to the control system 26 over a suitable
data/address bus 42. The memory circuit 40 allows certain control
parameters, used by the control system 26 in controlling the
operation of the pacemaker, to be programmably stored and modified,
as required, in order to customize the pacer's operation to suit
the needs of a particular patient. Such data includes the basic
timing intervals used during operation of the pacemaker, such as
the programmed atrial escape interval (AEI). Further, data sensed
during the operation of the pacer may be stored in the memory 40
for later retrieval and analysis.
A telemetry circuit 44 is further included in the pacer 10. This
telemetry circuit 44 is connected to the control system 26 by way
of a suitable command/data bus 46. In turn, the telemetry circuit
44, which is included within the implantable pacer 10, may be
selectively coupled to an external programming device 48 by means
of an appropriate communication link 50, which communication link
50 may be any suitable electromagnetic link, such as an RF (radio
frequency) channel. Advantageously, through the external programmer
48 and the communication link 50, desired commands may be sent to
the control system 26. Similarly, through this communication link
50 and the programmer 48, data (either held within the control
system 26, as in a data latch, or stored within the memory 40), may
be remotely received from the pacer 10. In this manner,
non-invasive communications can be established from time to time
with the implanted pacer 10 from a remote, non-implanted, location.
See, e.g., U.S. Pat. No. 4,847,617, issued to Silvian, entitled
"High Speed Digital Telemetry System for Implantable Devices,"
incorporated herein by reference.
The pacer 10 in FIG. 1 is referred to as a dual-chamber pacemaker
because it interfaces with both the atria and the ventricles of the
heart. Those portions of the pacer 10 that interface with the
atria, e.g., the lead 14, the P-wave sense amplifier 22, the
A-pulse generator 18, and corresponding portions of the control
system 26, are commonly referred to as the atrial channel.
Similarly, those portions of the pacer 10 that interface with the
ventricles, e.g., the lead 16, the R-wave sense amplifier 24, the
V-pulse generator 20, and corresponding portions of the control
system 26, are commonly referred to as the ventricular channel.
In accordance with one embodiment of the present invention, the
pacemaker 10 may further include one or more physiological sensors
52 that is connected to the control system 26 of the pacer over a
suitable connection line 54. While the sensor 52 is illustrated in
FIG. 1 as being included within the pacer 10, it is to be
understood that the sensor may also be external to the pacer 10,
yet still be implanted within or carried by the patient. A common
type of sensor is an activity sensor, such as a piezoelectric
crystal, mounted to the case of the pacemaker. Other types of
sensors, such as physiologic sensors that sense the oxygen content
of blood, respiration rate, pH of blood, and the like, may also be
used in lieu of, or in addition to, an activity sensor. The type of
sensor, if any, used is not critical to the present invention. Any
sensor or combination of sensors capable of sensing body motion or
a physiological parameter relatable to the rate at which the heart
should be beating can be used. A pacemaker using such sensors is
commonly referred to as a "rate-responsive" pacemaker because such
a pacemaker adjusts the rate (escape interval) of the pacer in a
manner that tracks the physiological needs of the patient.
Referring next to FIG. 2, a block diagram of one embodiment of the
control circuit or system 26 of the pacer 10 is illustrated. It is
noted that other embodiments of a control system 26 may also be
utilized, such as a microprocessor-based control system. A
representative microprocessor-based system is described, for
example, in U.S. Pat. No. 4,940,052, entitled "Microprocessor
Controlled Rate-Responsive Pacemaker Having Automatic Threshold
Adjustment." The '052 patent is assigned to the same assignee as is
this application, and is incorporated herein by reference.
The control system shown in FIG. 2 is based on a state machine
wherein a set of state registers 60 define the particular state of
the pacer at any instant in time. In general, and as an overview of
state machine operation, each state, by design, causes a certain
activity or function to be carried out. Several states are executed
in a sequence during a given cardiac cycle. The sequence of states
that is executed in a particular cardiac cycle is determined by the
particular events that occur, such as the sensing of a P-wave or an
R-wave, as well as the current state, as certain states can only be
entered from certain other states. Only one state can exist at any
instant of time, although several different state machines (or
control systems) may operate in parallel to control diverse
functions. For example, the telemetry circuit 44 (FIG. 1)
preferably utilizes its own state machine, such as is described in
the above-cited patent. The telemetry circuit state machine
operates essentially independent of the control system state
machine of FIG. 2.
At the heart of the control system 26 is the state logic 62. It is
the state logic that controls the "state" of the state registers
60, and hence the function or operation that will next be carried
out by the system. The state logic 62 receives as inputs the
current state of the state registers, made available over a state
bus 64 (which state bus directs the state of the system to several
sections of the control system), as well as other signals
indicating the current status of the system or events that have
occurred. The output signals from the P-AMP 22 (FIG. 1) and the
R-AMP 24 (FIG. 1) are directed to an input decode logic circuit 66.
This circuit generates appropriate logic signals "IPW" (Inhibiting
P-Wave) and "IRW" (Inhibiting R-Wave) that are selected by a
multiplexer 68 and sent to rate-determining logic 70. These signals
are also sent to the state logic 62. The function of the
rate-determining logic 70 is to determine the rate at which either
the IPW or IRW signals are occurring. A signal representative of
this rate is sent, as an output signal from the rate determining
logic 70, to the state logic 62 over signal line 72.
Rate-determining logic 70 further receives a sensor rate signal
from the sensor 52 (FIG. 1), and (depending upon the particular
state of the system, as defined by the state registers 60, and as
made available to the rate-determining logic 70 over the state bus
64) sends a rate signal to the state logic 62 over signal line 72
indicative of this sensor rate.
Still referring to FIG. 2, a memory control circuit 74 provides the
needed interface between the circuits of the control system 26 and
the memory 40 (FIG. 1). This memory control circuit may be any
conventional memory access circuit that sends or receives data to
or from memory at a specified address. Data retrieved from memory
40 may be sent to either the state logic 62 (over signal line(s)
75) or to one or more programmable timers 76 (over signal line(s)
77). Data sent to memory 40 may be either the current state of the
system (obtained off of the state bus 64), or other selected
signals from the state logic (as made available over signal line(s)
78).
The programmable timer 76 defines a prescribed time interval, the
length of which is set by the signal(s) received from the memory
control 74 over signal line(s) 77, and the starting point of which
begins coincident with the start of the current state, as obtained
from the state bus 64. The timer 76 further generates a time-out
(T.O.) signal when this prescribed time interval has elapsed.
During the prescribed time interval, the timing function may be
reset by a reset signal, typically obtained from the input decode
logic 66, although some states (as obtained from the state bus 64)
may also effectuate an immediate reset of the timer 76. The
time-out signal is sent to time-out decode logic 78. It is the
function of the time-out decode logic to generate the appropriate
trigger signals that are sent to the A-pulse generator 18 or the
V-pulse generator 20 (FIG. 1). Further, an appropriate logic
signal(s) is sent to the state logic 62 by the time-out decode
logic 78 over signal line(s) 80 in order to notify the state logic
that the respective trigger signals have been generated. It is to
be understood that while FIG. 2 only shows one programmable timer
76, several such programmable timers may be used, as is required,
in order to simultaneously keep track of multiple time
intervals.
An oscillator 82, preferably a crystal-controlled oscillator,
generates a basic clock signal C0 that controls the operation of
the system logic. This clock signal C0 is sent to clock logic
circuits 84, where other appropriate clock signals, such as clock
signals C1, C2 and C3, are generated, all derived from the basic
clock signal C0. These clock signals are distributed throughout the
control system 26 in order to appropriately synchronize the various
events and state changes that occur within the pacemaker. The rate
of the basic clock signal C0 is not critical to the present
invention. In general, a rate of 25-40 Khz for the basic clock rate
C0 is adequate. This rate provides a basic time increment of 25-40
microseconds each clock cycle, and this is more than enough time to
effectively control the pacemaker operation. If desired, a faster
basic clock rate can be used, particularly by the memory control
74, to speed up the data transfer between the control system 26 and
the memory 40, although for most pacemaker operations, a fast data
transfer rate is not essential.
In operation, the control system of FIG. 2 starts at an initial
state, wherein the state registers 60 assume a prescribed value
that defines the initial state. For example, assuming four flip
flops are used for the state registers 60, an initial state might
be "1000" (hexadecimal "8") wherein the first flip flop assumes a
"1" state, and the remaining three flip flops each assume a "0"
state. This state may be defined as a V-A Interval (VAI) state
wherein a prescribed ventricular-to-atrial (V-A) interval is
initiated. For purposes of the present invention, this V-A interval
may be considered as the "atrial escape interval," or "AEI." As
soon as the memory control 74 detects that the VAI state has been
initiated, as evidenced by the "1000" appearing on the state bus
64, it retrieves from the memory 40 an appropriate data word,
previously programmed into the memory 40 from the external
programmer 48, or otherwise generated by the state logic 62, that
defines the desired length of the AEI. This data word is sent to
the programmable timer and sets the length of the time period that
is to be measured during the VAI state.
The timer 76 is essentially just a counter that counts down (or
counts up), using a specified clock signal, to the value specified
in the data word. When the counting has been completed, and
assuming that the counter has not been reset by the occurrence of a
P-wave or other sensed event, the counter or timer 76 is said to
have "timed-out," and an appropriate time-out signal is generated
and sent to the time-out decode logic 78. The decode logic, in
turn, recognizes that the current state of the system is the VAI
state (as determined by monitoring the state bus 64), and therefore
that the AEI has timed-out without any cardiac activity having been
sensed. Hence, an A-pulse trigger signal is generated and sent to
the A-pulse generator 18, so that the atrium can be stimulated. At
the same time, an appropriate logic signal(s) is sent to the state
logic 62 over the signal line(s) 80 to alert the state logic to the
fact that the timer 76 has timed-out.
The state logic 62, in response to receiving the signal(s) from the
time-out decode logic 78, and also in response to the current VAI
state, triggers the next state of the prescribed sequence. For DDD
operation, this state is typically a blanking state, or BLANK
state, during which the P and R sense amplifiers, 22 and 24, are
disabled. Accordingly, the state logic generates appropriate
signal(s) on signal lines 36 and 38 to blank the P-wave sense
amplifier 22 and the R-wave sense amplifier 24, and also causes the
state registers 60 to change to a BLANK state, which state could be
defined, for example, by the flip flops of the state registers 62
assuming a "0001" (hex "1") condition. This BLANK state, detected
on the state bus 64, causes the memory control circuitry to
retrieve an appropriate data word from memory that defines the
length of the blanking interval, which data word is loaded into the
programmable timer 76. As soon as the timer 76 times out,
indicating that the prescribed blanking interval has elapsed, a
time-out signal is generated that is sent to the time-out decode
logic 78. Upon receipt of this time-out signal, and in response to
the current state being a BLANK state, the time-out decode logic 78
sends an appropriate logic signal to the state logic 62. The state
logic 62 responds by steering the state registers 62 to assume the
next state in the prescribed sequence, which may be, for example,
an AV-Interval state.
At the beginning of the AV-Interval state, another value is loaded
into the programmable timer 76, or into an equivalent programmable
timer, that defines the length of the pacemaker-defined AV
interval, or "AVI." If the timer 76 times out without being reset,
indicating that no R-wave has been sensed, the decode logic
generates a V-pulse trigger signal, and notifies the state logic 62
of this event. The state logic, in turn, causes the next
appropriate state to be entered, which state may be another
blanking state, or BLANK state, similar to the one described above,
but having perhaps a different duration. At the conclusion or
timing out of this second BLANK state, the next state in the
prescribed sequence is initiated, which state may be a refractory
(REF) state.
In the manner described above, the control system 26 assumes one
state after another, thereby controlling the operation of the
pacemaker. In general, a state is changed when the timer 76, or an
equivalent timer, times out, or when a prescribed event occurs.
Further, in accordance with the present invention, if a prescribed
event occurs, e.g., the occurrence of a P-wave, then the next state
may be a PV-Interval state. The PV-Interval state is the same as
the AV-Interval state, described above, except that a different
value is loaded into the programmable timer 76, which different
value defines the length of the PV interval, or "PVI."
It is noted that the state of the control system could also be
changed by receipt of an appropriate command from the telemetry
system.
The control system 26 of FIG. 2 may be realized using dedicated
hardware circuits, or by using a combination of hardware and
software (or firmware) circuits. The appropriate sequence of states
for a given mode of operation, such as DDD (dual-chamber pacing,
dual-chamber sensing, dual mode (inhibited and triggered)); DDDR
(dual-chamber pacing, dual-chamber sensing, dual mode (inhibited
and triggered), rate-responsive); or VDI (ventricular chamber
pacing, dual-chamber sensing, inhibited mode), for example, can be
defined by appropriate control of the memory control 74 and the
state logic 62. These circuit elements, in turn, are most easily
controlled through an appropriate software or firmware program that
is placed or programmed into the pacemaker memory circuits. The
manner of accomplishing such programming is known in the art.
A detailed description of the various circuits of the control
system 26 of FIG. 2 will not be presented herein because all such
circuits may be conventional, or may be patterned after known
circuits available in the art. Reference is made, for example, to
U.S. Pat. No. 4,712,555, wherein a state machine-type of operation
for a pacemaker is described; U.S. Pat. No. 4,788,980, wherein the
various timing intervals used within the pacemaker and their
interrelationship are more thoroughly described; and U.S. Pat. No.
4,944,298, wherein an atrial rate-based programmable pacemaker is
described, including a thorough description of the operation of the
state logic used to control such a pacemaker. The '555, '980 and
'298 patents are incorporated herein by reference.
Of primary significance to the present invention is the manner in
which the AV interval (or PV interval) is adaptively adjusted as a
function of the measured natural conduction time of the patient.
The manner in which this is done is illustrated in the flowchart of
FIG. 3. In FIG. 3, as well as the other flowcharts presented
herein, each main step of the method being described is illustrated
as a "box" or "block." Reference numerals are assigned to each
block of the flowchart to aid in the description of the invention
that follows. Each step of the method, i.e., each block, may be
readily carried out by those of skill in the art by programming
appropriate "code" in the memory 40, which code causes the
necessary control signals to be generated to carry out the desired
steps. Equivalent techniques for generating the control signals
needed to carry out the prescribed method or sequence may also, of
course, be used.
As seen in FIG. 3, the method starts by setting the initial values
needed by the pacemaker to carry out DDD or DDDR pacing (block
102). Such values are, for the most part, no different than those
used when performing conventional DDD or DDDR pacing, and include
such values as an initial pacing rate (from which an appropriate
atrial escape interval is determined), an initial value for the AV
interval, blanking period values, maximum pacing rate values,
stimulation pulse amplitudes and widths, and the like. In
accordance with the present invention, such initial values also
include a minimum and maximum value for the AV (or PV) interval,
plus a prescribed time difference between the natural conduction
time of the patient and the pacemaker-defined AV (or PV) interval.
In some embodiments of the invention, it may also be important to
specify the difference between the AV interval and a PV interval,
where the AV interval is the natural conduction time as measured
from the delivery of an A-pulse to the subsequent occurrence of an
R-wave, and the PV interval is the natural conduction time as
measured from the occurrence of a P-wave to the subsequent
occurrence of an R-wave.
Once the initial values needed to carry out DDD or DDDR pacing have
been set, the specified DDD or DDDR pacing is carried out (block
104) in conventional manner, one cardiac cycle at a time, using the
programmed values. At some point in a cardiac cycle associated with
such DDD or DDDR pacing, an R-wave will occur; or a number of
consecutive cardiac cycles will go by without the occurrence of an
R-wave. Either event signals a need to determine the natural
conduction time of the patient (block 106), so that an appropriate
adjustment to the AV (or PV) interval of the pacemaker can be made,
as needed (block 108).
The occurrence of an R-wave indicates the depolarization of the
ventricles as a result of a natural or native conduction time that
is shorter than the presently existing AV (or PV) interval of the
pacemaker. Hence, such event indicates that the pacemaker-defined
AV (or PV) interval needs to be decreased. Accordingly, as soon as
an R-wave occurs, the natural or native conduction time of the
patient, t.sub.AR or t.sub.PR, is determined. Such natural
conduction time is determined as the time interval between the most
recent atrial activity, which would be either a P-wave or an
A-pulse, and the R-wave. That is, the native or natural conduction
time (note, as used herein, "native" and "natural" are used as
synonyms) begins with the occurrence of atrial activity, and ends
with the occurrence of an R-wave. If the most recent atrial
activity was a P-wave, then the conduction time measured is
t.sub.PR. If the most recent atrial activity was an A-pulse, then
the conduction time measured is t.sub.AR.
If an R-wave fails to occur for a prescribed number of cardiac
cycles, then that provides an indication that perhaps the natural
conduction time has increased, and that there is a need to increase
the AV (or PV) interval so that it is not too different than the
natural conduction time.
In either event, once a determination is made that the natural
conduction time has either decreased or increased (block 106), the
AV (or PV) interval of the pacemaker is then set to a value that is
just less than the determined natural conduction time. This is done
by either decreasing the AV (or PV) interval when it appears that
the natural conduction time has decreased (as is most often the
case when an R-wave has been sensed), or by increasing the AV (or
PV) interval when it appears that the natural conduction time may
have increased (as is most often the case when an R-wave has not
been sensed for a prescribed number of cardiac cycles).
After the AV (or PV) intervals have been set to be less than the
determined conduction time t.sub.AR (or t.sub.PR) at block 108,
then a determination is made as to whether DDD or DDDR pacing is to
continue (block 110). If not, then the method terminates (block
112). If so, then the method continues (block 104) by performing
the DDD or DDDR pacing for the next cardiac cycle using the
adjusted values of the AV (or PV) interval.
Turning next to FIG. 4, a more detailed flowchart is illustrated
that shows the preferred technique for determining or measuring the
natural conduction time of the patient (block 106 in FIG. 3), and
adjusting the AV (or PV) intervals accordingly (block 108 in FIG.
3).
In FIG. 4, the programmed values needed to carry out DDD or DDDR
pacing are programmed into the pacemaker in conventional manner
(block 120). In accordance with the present invention, such
programmed values include the number of cardiac cycles that must
occur without an R-wave before the AV (or PV) interval is
increased, the amount of such increase, an initial value for the
natural conduction time t.sub.AR (or t.sub.PR), or an indication of
a technique for determining such initial values, the difference X
and/or Y between the natural conduction times and the AV (or PV)
intervals, and the like (block 122). Once the initial values of
t.sub.AR or (t.sub.PR) have been determined, then the value of the
AV (or PV) interval is set to be a specified amount less than
t.sub.AR or t.sub.PR (block 124).
With the AV (or PV) interval set to an initial value, the DDD or
DDDR pacing cycle commences using such value, plus the other
programmed values (block 126). If an R-wave is sensed during the
pacing cycle (block 126), then that signals that the natural
conduction time t.sub.AR (or t.sub.PR) is shorter than the
pacemaker-defined AV (or PV) interval. The occurrence of the R-wave
indicates the end of the conduction time t.sub.AR (or t.sub.PR),
and thus permits a measurement of t.sub.AR (or t.sub.PR) to be
completed (block 130). Two different measurement techniques for
determining t.sub.AR (or t.sub.PR) are detailed more fully in FIGS.
5 or 6. The measured value of t.sub.AR (or t.sub.PR) is then used
as a basis for decreasing the AV (or PV) interval (block 132). The
AV interval is set to t.sub.AR -X, where X is a parameter having a
programmable value, a fixed value, or an adaptive value based on a
percentage of the heart rate. Similarly, the PV interval is set to
t.sub.PR -Y, where Y is a parameter having a programmable value, a
fixed value, or an adaptive value based on a percentage of the
heart rate.
As is described more fully below in conjunction with FIGS. 5 and 6,
in some embodiments of the invention, t.sub.PR and t.sub.AR are
measured separately, and separate values are programmed or
otherwise determined for the parameters X and Y. Thus, in such
embodiments, t.sub.PR and the resulting PV interval, and t.sub.AR
and the resulting AV interval, are totally independent of each
other. In other embodiments, one of t.sub.AR or t.sub.PR is
determined, whichever happens to occur first, and the other is
computed as a function of the measured value. In such embodiments,
t.sub.AR is set to be a prescribed number of milliseconds greater
than t.sub.PR. In such embodiments, there is thus a prescribed
relationship between t.sub.AR and t.sub.PR and the resulting AV and
PV intervals. For most purposes relating to the description of the
present invention, one of the AV (or PV) intervals, or one of the
conduction times t.sub.AR (or t.sub.PR), is all that is expressly
referenced, and it is assumed that the other can be determined in
an appropriate manner.
After the AV (or PV) interval has been set to its new value based
on the most recent measured value of t.sub.AR (Or t.sub.PR) (block
132), a determination is made as to whether the new value of the AV
(or PV) interval is less than or equal to a programmed minimum
value for the AV (or PV) interval, AV.sub.MIN (or PV.sub.MIN)
(block 134). If so, then the AV (or PV) interval is set to
AV.sub.MIN (or PV.sub.MIN). If not, then the AV (or PV) interval
maintains the value previously determined. If DDD (or DDDR) pacing
is to continue (block 138), then the next cycle of such pacing
continues using the newly set value of the AV (or PV) interval
(bock 126).
Should an R-wave not occur during the pacing cycle (block 128),
then a determination is next made (block 140) as to whether a
prescribed (programmed) number of cardiac cycles have occurred
without the occurrence of an R-wave. If not, then the next cycle
begins (block 126). If yes, then that indicates that perhaps the
natural conduction time has increased, and that the AV (or PV)
interval should also be increased to keep the difference between
such natural conduction time and the AV (or PV) intervals to a
minimum. Accordingly, the AV (or PV) interval is increased by a
prescribed amount, Z (block 142). The value Z may be a fixed value,
a programmable value, an adaptive value based on a percentage of
heart rate, or a value based on the current AV interval. After the
AV (or PV) interval has been increased, a determination is made as
to whether the new value of the AV (or PV) interval is greater than
or equal to a programmed maximum value for the AV (or PV) interval,
AV.sub.MAX (or PV.sub.MAX) (block 144). If so, then the AV (or PV)
interval is set to AV.sub.MAX (or PV.sub.MAX). If not, then the AV
(or PV) interval maintains the value previously determined (at
block 142). If DDD (or DDDR) pacing is to continue (block 138),
then the next cycle of such pacing continues using the newly set
value of the AV (or PV) interval (bock 126).
The number of cardiac cycles that must occur without the occurrence
of an R-wave before the AV (or PV) interval is increased is
preferably a programmable number, and may typically be anywhere
from 8 to 128 cycles. Alternatively, a specific time interval may
be specified, 2-10 minutes, that must elapse without the occurrence
of an R-wave before the AV (or PV) interval is increased. The
amount Z by which the AV (or PV) interval is incrementally
increased is also preferably a programmable value, but could be a
fixed value, or an adaptive value. Typical values for Z range from
5-30 msec.
Referring next to FIG. 5, a flowchart is shown that illustrates one
technique for measuring the natural conduction times, t.sub.AR and
t.sub.PR, during one or more cardiac cycles of the heart. The
technique shown in FIG. 5 makes an independent measurement of both
t.sub.AR and t.sub.PR. As seen in FIG. 5, at the beginning of the
cardiac cycle (block 150), a PR timer and an AR timer are reset
(block 152). Such timers, as well as the other timers referenced
herein, may be implemented in hardware or software within the
control system 26 (FIGS. 1 and 2).
After resetting such timers, an atrial escape interval (AEI) begins
(block 154). If a P-wave is not sensed during the AEI (blocks 156,
174), then an A-pulse is generated (block 158), and the AR timer
commences (block 160). Also, the AV interval begins (block 162). If
an R-wave occurs during the AV interval (blocks 164, 166), then the
AR timer is stopped, and the value of the AR timer represents a
measure of the conduction time t.sub.AR (block 168). An AR flag is
then set (block 170), and the cardiac cycle ends (block 172),
having determined t.sub.AR during the cycle.
If the AV interval times out without detecting an R-wave (block
164), then a V-pulse is generated (block 188), and the cardiac
cycle ends (block 172), having made no determination of either
t.sub.AR or t.sub.PR during the cycle. Thus, the value of t.sub.AR
and/or t.sub.PR used at the beginning of the next cardiac cycle is
retained as the conduction time value used for the preceding
cardiac cycle.
Should a P-wave be sensed before the AEI times out (blocks 156,
174), than the PR timer is started (block 176). Also, the PV
interval is started (block 178). If an R-wave occurs during the PV
interval (blocks 189, 182), then the PR timer is stopped, and the
value of the PR timer represents a measure of the conduction time
t.sub.PR (block 184). A PR flag is then set (block 186), and the
cardiac cycle ends (block 172), having determined t.sub.PR during
the cycle.
It is noted that the AR and PR flags that are set during the
cardiac cycle, depending upon whether a P-wave or an A-pulse
occurs, may be used during the operation of the pacemaker to steer
the adjustment of the AV interval (if the AR flag is set), or the
PV interval (if the PR flag is set).
Turning next to FIG. 6, a flowchart of another embodiment or
technique for determining the natural conduction time of a patient
is illustrated. The technique shown in FIG. 6 determines just one
of t.sub.AR or t.sub.PR, and the other is set as a prescribed
difference from the measured value. Thus, as seen in FIG. 6, once
the cardiac cycle begins (block 180), a single timer, designated as
the A/P-R Timer, is reset, as is a single flag, designated the
A-Flag (block 182). The atrial escape interval (AEI) is started
(block 184), and a determination is made as to whether a P-wave is
sensed before the timing-out the AEI (blocks 186, 188). If so, then
the A/P-R Timer is started (block 190), and the PV interval is
started (block 192). While the PV interval is timing-out, a
determination is made as to whether an R-wave occurs (blocks 194,
196). If an R-wave does occur during the PV interval, then the
A/P-R Timer is stopped (block 214), and a determination is made as
to whether the A-Flag is set (block 216). If the A/P-R Timer is not
set, then that signals that the A/P-R Timer contains the t.sub.PR
value, which t.sub.PR value may be read from the A/P-R Timer, and
the t.sub.AR value may be computed therefrom. Typically, t.sub.AR
is computed as the measured value of t.sub.PR less Y.sub.A msec,
where Y.sub.A may be a fixed value, a programmed value, or an
adaptive value based on a percentage of the heart rate. The cardiac
cycle is then completed (block 200) having measured a value of
t.sub.PR and computed a value of t.sub.AR during the cycle.
If the PV interval times out without sensing an R-wave (blocks 194,
196), then a V-pulse is generated (block 198), and the cardiac
cycle terminates (block 200) without having determined a new value
for the conduction time t.sub.AR or t.sub.PR. Hence, the next
cardiac cycle starts using the previously determined values of
t.sub.AR or t.sub.PR.
If the AEI times out without having sensed a P-wave (blocks 186,
188), then the A-Flag is set (block 202), and an A-pulse is
generated (block 204). Also, the A/P-R Timer is started (block
206), and the AV interval is started (block 208). While the AV
interval is timing-out, a determination is made as to whether an
R-wave occurs (blocks 210, 212). If an R-wave does occur during the
AV interval, then the A/P-R Timer is stopped (block 214), and a
determination is made as to whether the A-Flag is set (block 216).
If the A-Flag is set, then that signals that the A/P-R Timer
contains the t.sub.AR value, which t.sub.AR value may be read from
the A/P-R Timer, and the t.sub.PR value may be computed therefrom.
Typically, t.sub.PR is computed as the measured value of t.sub.AR
minus Y.sub.B msec, where Y.sub.B may be a fixed value, a
programmed value, or an adaptive value based on a percentage of the
heart rate. The cardiac cycle is then completed (block 200) having
measured a value of t.sub.AR and computed a value of t.sub.PR
during the cycle.
If the AV interval times out without sensing an R-wave (blocks 210,
212), then a V-pulse is generated (block 198), and the cardiac
cycle terminates (block 200) without having determined a new value
of the conduction times t.sub.AR or t.sub.PR. Hence, the next
cardiac cycle starts using the previously determined values of
t.sub.AR Or t.sub.PR.
Thus it is seen that the present invention provides an implantable
pacemaker, and method of operating such a pacemaker, that
stimulates cardiac tissue at a time in the cardiac cycle that is
just prior to when natural depolarization of the cardiac tissue
would otherwise cause a cardiac contraction.
As further described above, it is seen that the invention provides
a dual-chamber pacemaker, and method of operating such a
dual-chamber pacemaker, that automatically adjusts the
pacemaker-defined AV interval to a value that is just less than the
natural conduction time of the patient. Such action advantageously
assures that a V-pulse is generated and delivered to the
ventricular muscle tissue at a time in the cardiac cycle when such
ventricular muscle is not refractory (i.e., prior to the natural
depolarization of the ventricular tissue), while still maintaining
the approximate cardiac timing set by the natural conduction time,
thereby maximizing the cardiac output of the patient.
As also described above, it is seen that the invention provides a
dual-chamber pacemaker, and method of operating such a pacemaker,
that decreases the pacemaker-defined AV interval in response to
sensing an R-wave (which sensed R-wave evidences a shortened
natural conduction time), and that automatically increases the
pacemaker-defined AV interval in prescribed increments in response
to not sensing an R-wave for a prescribed number of consecutive
cardiac cycles (which failure to sense any R-waves may evidence a
lengthening of the natural conduction time). Thus, advantageously,
the pacemaker-defined AV interval is most always set to a value
that is just somewhat less than the natural conduction time,
regardless of whether the natural conduction time is increasing or
decreasing, and the cardiac output of the patient is maximized.
While the invention herein disclosed has been described by means of
specific embodiments and applications thereof, numerous
modifications and variations could be made thereto by those skilled
in the art without departing from the scope of the invention set
forth in the claims.
* * * * *